Active brake unit

ABSTRACT

The invention relates to a circuit arrangement for recirculating the energy produced during the braking of electric motors into a supply system. The object of the invention is to implement the feeding of the energy that can be obtained when braking electric motors to the supply system without the use of an isolating transformer. Said object is solved by a forward branch ( 3 ), comprising a rectifier ( 32 ) connected to the supply system ( 1 ). The rectifier is guided via a first intermediate circuit ( 33 ) to a first inverter ( 34 ) that is connected to the motor ( 2 ), and a backward branch ( 4 ), comprising a second intermediate circuit ( 42 ) connected to the output of the first intermediate circuit ( 33 ), wherein a second inverter ( 41 ) is connected to the second intermediate circuit, and the second inverter in turn is connected to the supply system ( 1 ) via a mains circuit ( 5 ). Each pole of the second intermediate circuit ( 42 ) is connected via a series connection of a current-compensated throttle ( 61, 62 ) and a diode ( 64, 65 ) to the output of the first intermediate circuit ( 33 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a circuit arrangement for recirculating theenergy produced during the braking of electric motors into a supplysystem. Preferred areas of application for devices of this type includethe drives of machine tools and textile, printing and other machines forwhich frequent starting up and braking is characteristic.

2. Description of the Related Art

Devices for recirculating the energy produced during the braking ofelectric motors are known in the art. One such device is described in“Unidrive Regen Installation Guide, Issue No. 2” on the websitehttp://www.controltechniques.com. This involves an adaptation ofcontrolled direct current power supplies to other drives. The device iscomprised of a power supply branch, which is made up of avoltage-controlled resistive circuit that is connected to the powersupply system and an HF filter, a forward branch and a reverse branch.An electric motor is connected to the output side of the forward branch.The forward branch comprises a rectifier, an intermediate circuit and aninverter. The output terminals of the intermediate circuit are alsoconnected via a blocking diode to the reverse branch, which comprises asecond inverter and an adapting circuit with a line filter, saidadapting circuit being connected via one choke per phase, and anisolating transformer, which is connected to the power supply branch.The isolating transformer effects the galvanic separation of the reversebranch from the supply system, thereby preventing the creation of highcirculating currents. However, the isolating transformer has arelatively high volume and represents a major cost factor.

The object of the invention is therefore to implement the feeding of theenergy that can be obtained during the braking of electric motors to thesupply system without the use of an isolating transformer.

SUMMARY OF THE INVENTION

The object is obtained with an active braking unit having thecharacterizing features of patent claim 1, in that a forward branch anda reverse branch are connected to the supply system. The forward branchconsists of a rectifier, which is connected to the supply system and isguided via a first intermediate circuit to a first inverter, which isconnected to the motor. The reverse branch comprises a secondintermediate circuit connected to the output of the first intermediatecircuit, wherein a second inverter is connected to the secondintermediate circuit, and wherein each pole of the second intermediatecircuit is connected via the series connection of a current-compensatedchoke and a diode to the output of the first intermediate circuit. Thereverse branch is connected to the supply system via a system adaptingcircuit.

One advantageous improvement on the invention is presented in thedependent claim, wherein the output voltage of the second inverter iscontrolled based upon the voltage via the second intermediate circuit.During load operation, the motor is connected to the supply system andthereby forms a sink in current, with speed and torque being adjusted bymeans of the forward circuit. During braking operation, the voltage ofthe first intermediate circuit increases, because the flow of current tothe motor is adjusted and a reverse current from the motor is generated.The reverse current can advantageously be returned to the intermediatecircuit by means of an inverter, which is comprised of insulated gatebipolar transistors (IGBT's), the emitters and collectors of which areconnected via freewheeling diodes. According to the invention, thecurrent-compensated chokes limit the current and the current increasewhich flow into the reverse branch as a result of the increase involtage at the first intermediate circuit. The voltage occurring at thesecond intermediate circuit is adjusted using the controlled secondinverter. In this way, the blocking diodes prevent a discharge of thefirst intermediate circuit via the current-compensated chokes, which areseries-connected to them, until a transmittance threshold for thediodes, based upon the voltage difference occurring between the firstand second intermediate circuits during braking operation, is exceeded.By means of the preferably controlled second inverter, the energy thathas then flowed into and been stored in the second intermediate circuitis recirculated to the supply system via the system adapting circuit. Inthis process, the reverse branch functions as a step-up converter,effecting an adaptation and control of the voltage obtained from thebraking energy with respect to the voltage of the supply system. Togenerate the sinusoidal current, the second inverter is actuated withpulse-width modulated (PWM) signals, so that the harmonics areminimized.

BRIEF DESCRIPTION OF THE DRAWING

In what follows, the invention will be specified in greater detail inthe form of a preferred exemplary embodiment in reference to theattached diagram. The diagram shows

-   -   a circuit diagram illustrating the principle of an active        braking unit according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

A motor 2 is connected via a forward branch 3 to a 3-phase system 1. Theforward branch 3 is comprised of one commutating coil or commutatingchoke 311, 312, 313 per phase, which are guided to a rectifier 32.Diodes or thyristors can be used as rectifier elements. The rectifier 32is connected via a first intermediate circuit 33 to a schematicallyrepresented first 3-phase inverter 34, the switching elements of whichare embodied as insulated gate bipolar transistors (IGBT) withfreewheeling diodes that are connected in parallel to theemitter-collector paths. The motor 2 is connected to the outputs of thefirst inverter 34. To return the braking energy that is produced duringbraking of the motor 2, a reverse branch 4 is provided, the outputs ofwhich are guided to the 3-phase system 1. The reverse branch 4 isconnected to the output of the first intermediate circuit 33 via twocoils 61, 62, which are magnetically coupled via a shared core 63 andare wound opposite one another, and which therefore function ascurrent-compensated chokes, and via diodes 64, 65, which are seriesconnected to the coils in the forward direction with respect to theterminals on the first intermediate circuit 33. The reverse branch 4 iscomprised of a second intermediate circuit 42 and a second inverter 41,which is identical in structure to the first inverter 34. Parallel tothe second intermediate circuit 42, a controller 7 is connected, theoutput of which is connected to the gate terminals of the insulated gatebipolar transistor (IGBT) of the second inverter 41. The intermediatecircuit voltage detected by the controller 7 at the second intermediatecircuit 42 is controlled to a setpoint value set at the input 71. Forthis purpose, according to another optional embodiment of the invention,as an alternative or in addition to the above, a return of the threeoutput phase currents from the second inverter 41 (which are conductedto HF chokes 51, 52, 53) to corresponding inputs of the controller 7 canbe provided as indicated in the drawing. The output terminals of thesecond inverter 41 are connected to the 3-phase system 1 via a systemadapting circuit 5, which consists of the series connection of one HFchoke 51, 52, 53 per phase. When the motor 2 is braked, the firstintermediate circuit voltage is increased, because the flow of currentto the motor 2 is adjusted, and a reverse current from the motor 2 isgenerated. The reverse current is returned to the first intermediatecircuit 33. The current-compensated chokes 61, 62 limit the currentflowing into the reverse branch 4 as a result of the voltage increase atthe first intermediate circuit 33, along with the corresponding currentincrease (for functioning see below). The voltage occurring at thesecond intermediate circuit 42 is adjusted by the controlled secondinverter 41. In this process, the blocking diodes 64, 65 prevent adischarge of the second intermediate circuit 42 via thecurrent-compensated chokes 61, 62 that are series connected to them, sothat the energy stored in the second intermediate circuit 42 isrecirculated via the second inverter 41 and the system adapting circuit5 into the 3-phase system 1. The reverse branch 4 functions as a step-upconverter, whereby an adjustment and control of the voltage obtainedfrom the braking energy is implemented with respect to the voltage ofthe 3-phase system 1. To generate the sinusoidal current, the secondinverter 41 is actuated with pulse-width modulated (PWM) signals, sothat the output-side harmonics are minimized.

At the capacitor of the first intermediate circuit 33 or at the outputof the rectifier 32 a voltage is present, which amounts to approximately1.35 times the system voltage (effective value). It is known in the artthat with the sinusoidal system voltage, the amplitude amounts to √2times the system effective voltage. √2 is approximately 1.4, and inpractical applications, the value 1.35 has proven effective because itapproximates the mean value of the output voltage from the rectifierwith residual ripple. The second intermediate circuit voltage at thecapacitor of the second intermediate circuit 42 is controlled via thecontroller 7. The step-up conversion takes place from the HF chokes 51,52, 53 to the second intermediate circuit 42. Therefore, the secondintermediate circuit voltage is higher than the voltage at the firstintermediate circuit 33. When the motor 2 is in drive status, energyfrom the system 1 is fed to the motor via the rectifier 32, the firstintermediate circuit 33 and the first inverter 34. At the same time,because of the blocking diodes 64, 65 the second intermediate circuit isuncoupled from the first intermediate circuit to the same extent. Whenthe motor 2 moves to a braking status, the occurring braking energy isfirst recirculated via the first inverter 34 into the first intermediatecircuit 33. The intermediate circuit voltage of the first intermediatecircuit 33 thus increases. Once the voltage in the first intermediatecircuit 33 becomes greater than the voltage in the second intermediatecircuit 42, the two blocking diodes connect through in the direction ofthe second inverter 41. It is thereby possible for the braking energythat was first stored in the first intermediate circuit 33 to berecirculated into the system via the two blocking diodes 64, 65 and thesecond inverter and via the HF chokes 51, 52, 53. If the voltage in thefirst intermediate circuit 33 is higher than 1.35 times the systemvoltage due to the returning braking energy, the rectifier 32 blocks thefirst intermediate circuit 33 from the system 1. An electric separationthus occurs. The blocking diodes 64, 65 then connect through to thesecond intermediate circuit 42.

Numerical example: The system voltage supplies an effective voltage of400 volts. The voltage at the capacitor in the first intermediatecircuit 33 therefore amounts to 1.35 times, or approximately 540 volts.At the controller input 71, the setpoint value is adjusted to 600 volts,for example. Only when these 600 volts are exceeded in the firstintermediate circuit 33 as a result of returning braking energy do theblocking diodes 64, 65 connect through and release the backflow ofbraking energy via the reverse branch 4.

LIST OF REFERENCE SYMBOLS

-   1 3-phase system-   2 Motor-   3 Forward branch-   311, 312, 313 Commutating chokes-   32 Rectifier-   33 First intermediate circuit-   34 First IGBT inverter-   4 Reverse branch-   41 Second IGBT inverter-   42 Second intermediate circuit-   5 System adapting circuit-   51, 52, 53 HF chokes-   61, 62 current-compensated chokes-   63 Shared coil core-   64, 65 Blocking diodes-   7 Controller-   71 Setpoint value input

1. Active braking unit for recirculating energy that is produced duringthe braking of electric motors (2) to a supply system (1) with a forwardbranch (3), comprising a rectifier (32) that is attached to the supplysystem (1), said rectifier being guided via a first intermediate circuit(33) to a first inverter (34), which is connected to the motor (2), anda reverse branch (4), comprised of a second intermediate circuit (42),which is connected to the output of the first intermediate circuit (33)and has a second inverter (41) connected to it, which second inverter isconnected via a system adapting circuit (5) and/or other system couplingmeans to the supply system (1), characterized in that each pole of thesecond intermediate circuit (42) is connected to the output of the firstintermediate circuit (33) via a series connection of acurrent-compensated choke (61, 62) and a diode (64, 65).
 2. Activebraking unit according to claim 1, characterized by a configuration ordimensioning such that when the motor is in driving status, the voltageof the second intermediate circuit (41) is greater than that of thefirst intermediate circuit (33).
 3. Active braking unit according toclaim 1, characterized in that the output voltage of the second inverter(41) is controlled based upon the voltage via the second intermediatecircuit (42).
 4. Active braking unit according to claim 1, characterizedin that the output voltage of the second inverter (41) is controlledbased upon the output current or currents of the second inverter (41).5. Active braking unit according to claim 4, characterized in that thecorresponding controller (7) has an input (71), which is supplied with asetpoint value, which provides a voltage of the second intermediatecircuit (42) that is higher than that of the first intermediate circuit(33) when the motor is in drive status.
 6. Active braking unit accordingto claim 5, characterized by an adjustability at the setpoint valueinput.
 7. Active braking unit according to claim 6, characterized inthat the second intermediate circuit (41) is implemented using anintermediate circuit capacitor, which is coupled to the firstintermediate circuit via the current-compensated chokes (61, 62) and theblocking diodes (64, 65).